The present invention relates to a process of controlling ramp imbalances in any number of ramp environments that have an inflow and outflow from one or more systems to one or more other systems, for example, petrochemical environments. In one embodiment, the present disclosure relates to controlling ramp imbalances in the process of liquefying a gaseous, methane-rich feed to obtain a liquefied product (liquefied natural gas).
The liquefaction process includes the steps of:
(a) supplying the gaseous, methane-rich feed at elevated pressure to a first tube side of a main heat exchanger at its warm end, cooling, liquefying and sub-cooling the gaseous, methane-rich feed against evaporating refrigerant to get a liquefied stream, removing the liquefied stream from the main heat exchanger at its cold end and passing the liquefied stream to storage as liquefied product;
(b) removing evaporated refrigerant from the shell side of the main heat exchanger at its warm end;
(c) compressing in at least one refrigerant compressor the evaporated refrigerant to get high-pressure refrigerant;
(d) partly condensing the high-pressure refrigerant and separating in a separator the partly-condensed refrigerant into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction;
(e) sub-cooling the heavy refrigerant fraction in a second tube side of the main heat exchanger to get a sub-cooled heavy refrigerant stream, introducing the heavy refrigerant stream at reduced pressure into the shell side of the main heat exchanger at its mid-point, and allowing the heavy refrigerant stream to evaporate in the shell side; and
(f) cooling, liquefying and sub-cooling at least part of the light refrigerant fraction in a third tube side of the main heat exchanger to get a sub-cooled light refrigerant stream, introducing the light refrigerant stream at reduced pressure into the shell side of the main heat exchanger at its cold end, and allowing the light refrigerant stream to evaporate in the shell side.
International patent application publication No. 99/31448 discloses controlling a liquefaction process by an advanced process controller based on model predictive control to determine simultaneous control actions for a set of manipulated variables in order to optimize at least one of a set of parameters whilst controlling at least one of a set of controlled variables. The set of manipulated variables includes the mass flow rate of the heavy refrigerant fraction, the mass flow rate of the light refrigerant fraction and the mass flow rate of the methane-rich feed. The set of controlled variables includes the temperature difference at the warm end of the main heat exchanger and the temperature difference at the mid-point of the main heat exchanger. The set of variables to be optimized includes the production of liquefied product. The process was considered to be advantageous because the bulk composition of the mixed refrigerant was not manipulated to optimize the production of liquefied product. However, controlling ramp imbalances associated with level and pressure has proven to be cumbersome.
Prior systems addressed the static problem by taking all the controlled variables (CV) and manipulated variables (MV) bounds and imposing a steady state constraint using a process model and optimizing the various objectives that are given to the process model. The objectives may be specified in terms of user-specified values or settings, for example, the set-points or real-time optimization (RTO) targets, or as economic functions which are optimized over the feasible space. The static problem returns a feasible steady state (or steady-state targets), which satisfies the objectives. In dynamic control, the various objectives are set to track these steady-state targets. Due to infeasibility, these steady-state target values of the CVs may differ from the user-specified values (for example, the set-points or RTO targets) or may have a higher economic cost than other possible values within the specified CV limits. This creates the possibility of high potential values within the CV limits which are statically-infeasible.
The steady-state targets are the values where the CVs will stay there unless perturbed. However, under real world conditions these steady-state targets do not remain static and thus the CVs are in transient most of the time due to, for example, noise and un-modeled disturbances. Prior systems, such as those described in the papers J. B. Rawlings, D. Bonne, J. B. Jorgense, A. N. Venkat and S. B. Jorgensen, “Unreachable setpoints in model predictive control,” IEEE Transactions on Automatic Control, vol. 53, no. 9, pp. 2209-2215, 2008 and J. Rawlings and R. Amrit, “Optimizing process economic performance using model predictive control,” Nonlinear Model Predictive Control, 2009, pp. 119-138, performed evaluations to determine if any high potential areas existed, opening up a possibility to extract higher cumulative benefits in terms of either vicinity from the user-specified setpoints or economic benefit.
Unstable or ramp behavior in advanced process control design often come from the incorporation of level control of tanks and/or accumulators. Several refining and chemical processes also exhibit unstable behavior, for example, temperature control in partial combustion catalytic crackers. Prior systems exerted very tight control on ramp imbalances which unnecessarily constrained the control problem. For example, prior model-based predictive control (MPC) technologies automatically added an additional constraint that the rate-of-change of the CV had to be zero to the end of the prediction horizon window. In prior MPC environments, specified CVs for a given tank may put too much constraint on production. For example, feeding a tank through an inflow pipe may not be hindered by any constraints but outflow from the tank to another destination tank may impose certain constraints. Each tank has a certain capacity or inventory that is altered by inflows to and outflows from the tank. Because of constraints placed on an inflow or outflow, an imbalance may occur that prevents the efficient operation of the entire system within the given environment. For example, supply from a tank may be stopped if the level of inventory of the tank fell below a certain CV limit even though stopping such supply would cause other systems to exceed or fall below their respective CV limits causing unnecessary delay and expense. Such rigid adherence of the MPC to ramp imbalances is not efficient. In prior systems, an operator may have been required to override the control by the MPC and manually operate the inflow and outflow of the system. Such intervention was sporadic, prone to errors, imposed heavy time burdens, created expenses, and in some circumstances was infeasible due to conditions or the particular environment. The present invention aims to provide temporary relaxation of the ramp imbalance constraints when within certain limits.